1. Field of the Invention
The present invention relates to techniques by which an attribute of each pixel in an image is determined.
2. Description of the Related Art
Apparatuses that extract character edges using a differential filter or the like then classify characters and elements other than characters to execute adaptive processing are known as image processing apparatuses that execute optimal image processing according to image characteristics. Furthermore, apparatuses that take halftone dots of printing as isolation amounts then extract the halftone dots by accumulating the isolation amounts within a predetermined area and classify characters and halftone dots to execute adaptive processing are common (for example, Japanese Patent Laid-Open No. H11-85978.
Furthermore, methods have been proposed that identify not only characters and halftone dots but also extract characters within halftone dots as low frequency edges (for example, Japanese Patent Laid-Open No. 2006-5806.
However, although there is a high extraction capability (accuracy) for halftone dots that express characters on a white background and printed photos and the like with the conventional technique described in Japanese Patent Laid-Open No. H11-85978, the extraction capability for characters and line drawings within halftone dots is low. The extraction capability for characters in halftone dots is improved when the character extraction capability is enhanced as an attempt to extract characters in halftone dots, but at the same time image edges in halftone dots may end up being misidentified, and as a result character processing is executed also on photo image areas of printed materials, which incurs reduced image quality.
To solve this, it is necessary to separately identify characters on a white background and characters in halftone dots and execute suitable processing on these respective characters.
The conventional technique described in Japanese Patent Laid-Open No. 2006-5806 was proposed to solve this issue, but there are several types of halftone dots and it does not satisfactorily separate halftone dots and characters with good accuracy. Therefore, depending on the halftone dot document, there is a problem of moire occurring.